There has long been a need in the surveillance field to be able to detect and identify the presence of RF signals to be able to locate either enemy forces or, for instance, perpetrators of crimes in which either of these entities are utilizing RF communications. In the case of battlefield scenarios, communications between combat units or between the troops themselves is often carried over two-way radio links. In terms of the perpetrators of crimes, both cellular phone and other two-way communication links provide the perpetrators with necessary communications for their purposes.
While there exists many airborne snooping devices that can monitor RF communications, pinpointing the source of transmission is oftentimes elusive. Moreover, while it is possible with signal recognition algorithms to be able to identify a particular signal source, it is only with difficulty that the location of the signal source can be rapidly ascertained due to its intermittent nature.
There is therefore a requirement to be able to monitor troop movements or the movements of perpetrators through intercepting their RF transmissions, preferably with listening devices which are spaced apart on the ground at known locations. The problem with such listening devices is that they are by their very nature battery-powered so that once deployed their longevity is determined by the power drain of the individual devices.
Not only are the listening devices constrained by power regimes, their ability to identify a particular transmission from all other electromagnetic radiation in the area is of importance so that the source of the transmission can be identified by its spectral signature. Moreover, identifying the existence of a signal of interest is insufficient by itself to give the geographic location of the signal source.
In the past, and as will be documented below, the spectral signature of the transmission can be monitored in terms of histograms so that the source itself can be identified accurately. Such systems require a spectrum analyzer to be able to ascertain the spectral components, including frequency and amplitude of the components of the intercepted signals.
However, conventional spectrum analyzers are in general not portable, and more particularly consume large amounts of power in order to provide spectral content with sufficient resolution to be able to make a determination of the particular source of the intercepted signal.
As illustrated in U.S. Pat. No. 4,305,159 issued to Chester E. Stromswold, John T. Apostolos, Robert T. Boland, and Walter J. Albersheim, assigned to the assignee hereof and incorporated herein by reference, a compressive receiver is described in which the traditional envelope detector is replaced with a Fourier transform device such that the output of the dispersive delay line utilized in the receiver is processed to yield the spectrum of incoming signals. As mentioned in this patent, in order to permit spectral analysis of many signals over wide bandwidths, an especially wide bandwidth dispersive delay line is required along with a sweep-to-sweep phase coherent variable frequency oscillator.
It will be appreciated that the compressive receiver described in the above-noted patent is one that can analyze only a single frequency bin at a time. This made the prior art compressive receivers both high-cost, high power consuming and slow due to the fact that only one frequency bin could be analyzed at a time.
Not only is this prior art compressive receiver slow, a major power consuming cost component is the sweep to sweep phase coherent variable frequency oscillator, referred to as a chirp generator. The purpose of the chirp generator is to provide frequency changes which match the frequency changes associated with the dispersive display line. Note that dispersive delay lines are dispersive in the sense that their delay changes with frequency.
When contemplating the utilization of a battery-operated field-deployed spectrum analyzer to listen for incoming signals and report their presence to a central location for surveillance purposes, the chirp generator is the device in the compressive receiver which utilizes the most energy. Dispersive delay lines are passive devices, and the gates utilized to provide the frequency bins are likewise extremely low-power devices.
On the other hand chirp generators typically draw 200 milliamps, and if simply left on for surveillance purposes would result in the running down of a battery in for instance, four hours.
Moreover, a spectrum analyzer associated with a single chirp generator would result in frequency bins of, for instance 20 megahertz. A 20 megahertz wide spectral line is very broad and offers very little in the way of a spectral fingerprint.
What this means is that the spectral content of an incoming signal cannot be adequately analyzed by a compressive receiver with a single dispersive delay line and a single chirp generator.
The ability to not only indicate the presence of an incoming signal, but also to be able to identify it by its spectral components is indeed important in the surveillance field. In battlefield conditions or other surveillance opportunities, it is important not only to know that there is an incoming signal, but also to be able to characterize it as to what type of signal it is so that its source can be identified. One way of identifying the origin of the signal is to utilize the technique described in U.S. Pat. No. 4,166,980 entitled: Method And Apparatus For Signal Recognition, by John T. Apostolos and Robert P. Boland, assigned to the assignee hereof and incorporated herein by reference. The system described by this patent utilizes histograms to be able to characterize the intercepted signal by the number of occurrences of their spectral lines, their frequency and their amplitudes.
In order to completely characterize the existence and identify an incoming signal, it is therefore important to be able to provide a spectral analysis of the incoming signal in terms of the amplitude and frequency components thereof. It is also important to be able to provide a spectrum analyzer which is indeed wideband so that whatever the frequency of the incoming signal is, it is analyzable by the spectrum analyzer. The resolution of the spectrum analyzer is likewise important.
For compressive receivers of the type described in U.S. Pat. No. 4,305,159, the degree of resolution for the spectral lines is determined by the characteristics of the dispersive delay line. With a single dispersive delay line in one instance the frequency bins are 20 megahertz wide.
This type of system is therefore incapable of determining the precise position of the spectral lines of the incoming signal.
By way of further background, for instance, if the spectrum analyzer listening devices and respective transmitters were dropped or placed along a path in a battlefield situation to listen for signal intelligence in the form of two-way radio traffic, in an unattended situation, it is important that the units operate over a sufficiently long period of time to provide adequate surveillance. Thus, for instance, with a typical chirp generator and with a lithium hydride battery of 0.8 amp hours, then it will be appreciated that the battery if fully charged at the beginning of the mission would run down in approximately four hours. This would give four hours worth of surveillance.
However, if the resolution of the spectrum analyzer is not sufficient to identify the incoming signals, then the relatively coarse frequency bins must be further analyzed by an additional spectrum analyzer in order to obtain the spectral signature of the incoming signal. This in turn virtually doubles the power consumption required such that the surveillance might only take place over two hours as opposed to four hours.
The above precludes spreading of a number of spectrum analyzer listening devices over a given battle or surveillance area to pick up and identify signals due to the high battery drain.
Moreover, the cost of a spectrum analyzer module of the type described in U.S. Pat. No. 4,305,159 is primarily dependent on the cost of the chirp generator. The dispersive delay line and the gate are relatively inexpensive items, but with the requirement of sweep-to-sweep phase coherence for the chirp generator, the generator is relatively costly. If one were to deploy for instance one hundred of these spectral analyzers over a battlefield or surveillance area, then the cost of so doing is considerable. Add to this the cost of additional spectrum analyzers to refine the results of the single chirp spectrum analyzer, then the cost clearly becomes prohibitive.